Converting a column containing a map into json throws an exception

[Feng-Jiang28]

Converting a column containing a Map into json throws an exception:

Reproduce:

CPU:

scala> val df = Seq(Map("a" -> Tuple1(1))).toDF("a")
scala> df.write.mode("OVERWRITE").parquet("TEMP")
scala> val df2 = spark.read.parquet("TEMP")
scala> df2.select(to_json($"a")).show()
+--------------+                                                                
|    to_json(a)|
+--------------+
|{"a":{"_1":1}}|
+--------------+

GPU:

$ $SPARK_HOME/bin/spark-shell --master local[*] --jars ${SPARK_RAPIDS_PLUGIN_JAR} 
--conf spark.plugins=com.nvidia.spark.SQLPlugin 
--conf spark.rapids.sql.enabled=true 
--conf spark.rapids.sql.explain=ALL --driver-java-options '-ea -Duser.timezone=UTC ' 
--conf spark.rapids.sql.expression.JsonTuple=true 
--conf spark.rapids.sql.expression.GetJsonObject=true 
--conf spark.rapids.sql.expression.JsonToStructs=true 
--conf spark.rapids.sql.expression.StructsToJson=true

scala> val df = Seq(Map("a" -> Tuple1(1))).toDF("a")
scala> df.write.mode("OVERWRITE").parquet("TEMP")
scala> val df2 = spark.read.parquet("TEMP")
df2: org.apache.spark.sql.DataFrame = [a: map<string,struct<_1:int>>]
scala> df2.select(to_json($"a")).show()
  @Partitioning <SinglePartition$> could run on GPU
  *Exec <ProjectExec> will run on GPU
    *Expression <Alias> to_json(a#24, Some(UTC)) AS to_json(a)#30 will run on GPU
      *Expression <StructsToJson> to_json(a#24, Some(UTC)) will run on GPU
    *Exec <FileSourceScanExec> will run on GPU
 ...
24/05/25 04:59:58 ERROR Executor: Exception in task 0.0 in stage 7.0 (TID 7)
java.lang.ClassCastException: org.apache.spark.sql.types.MapType cannot be cast to org.apache.spark.sql.types.StructType
	at org.apache.spark.sql.rapids.GpuStructsToJson.doColumnar(GpuStructsToJson.scala:86)
	at com.nvidia.spark.rapids.GpuUnaryExpression.doItColumnar(GpuExpressions.scala:250)
	at com.nvidia.spark.rapids.GpuUnaryExpression.$anonfun$columnarEval$1(GpuExpressions.scala:261)
	at com.nvidia.spark.rapids.Arm$.withResource(Arm.scala:30)
	at com.nvidia.spark.rapids.GpuUnaryExpression.columnarEval(GpuExpressions.scala:260)
	at com.nvidia.spark.rapids.RapidsPluginImplicits$ReallyAGpuExpression.columnarEval(implicits.scala:35)
	at com.nvidia.spark.rapids.GpuAlias.columnarEval(namedExpressions.scala:110)
	at com.nvidia.spark.rapids.RapidsPluginImplicits$ReallyAGpuExpression.columnarEval(implicits.scala:35)
	at com.nvidia.spark.rapids.GpuProjectExec$.$anonfun$project$1(basicPhysicalOperators.scala:110)
	at com.nvidia.spark.rapids.RapidsPluginImplicits$MapsSafely.$anonfun$safeMap$1(implicits.scala:221)
	at com.nvidia.spark.rapids.RapidsPluginImplicits$MapsSafely.$anonfun$safeMap$1$adapted(implicits.scala:218)
...